1 Development versions of the packages

2 Performance issues

Calculation of the the adjusted degrees of freedom for the
Kenward-Roger approximation can be computationally demanding because it
requires inversion of an N ×N matrix where N is the number
of observations.

Possible remedies for this:

On linux (ubuntu) I have observed that changing the BLAS to ATLAS-BLAS
generally provides a speed-up with a factor 3-6 (compared with R's
default BLAS). That helps in some situations.

Parametric bootstrap is an alternative, and while also
computationally intensive, parametric bootstrap can be parallelized
(facilities exist in pbkrtest).

Lastly, an alternative to Kenward-Roger is to implement a
Satterthwaites approximation. It is on the todo-list.